1. Field of the Invention
The present invention relates to apparatus for applying a coating fluid to a moving web, and particularly to coating apparatus of the type utilizing a slot die head.
2. Description of the Prior Art
It is known to apply coating fluids to moving webs by means of a slot die head for the purpose of achieving an accurate and smooth film of fluid on the web surface. Presently known slot die heads have a slot orifice from which the coating fluid is emitted onto the web. The outlet end of the orifice is defined by a pair of lips, which may be in the form of replaceable lip inserts removably attachable to the die head. One of the lips may be adjustable in directions substantially perpendicular to the slot axis for varying the orifice opening and thereby varying the smoothness and shearing characteristics of the sheet of fluid emitted from the slot orifice.
The lips respectively extend from the orifice upstream and downstream relative to the direction of web movement, the downstream lip serving as a wiping lip for controlling the thickness of the film coating on the web. In order to achieve proper smoothing of the coating fluid, the wiping lip must be positioned extremely close to the web surface to obtain the desired coating effect. The web is typically moved around a backup roll. During the wiping operation, variations in the system parameters such as the roll diameter, roll hardness, web thickness, lip wiping face straightness or pulsing action from the fluid pump will tend to induce differential shear levels into the coating fluid. These differential shear levels will change the smoothing characteristics of the fluid and therefore change the visual appearance of the coating. Also, differing shear levels across a web may induce variations in the performance characteristics of the adhesive-coated web, reducing the uniformity of performance in the end product.